15.2 Distances of Galaxies

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15.2 Distances of Galaxies

• Our Goals for Learning

How do we measure the distances to galaxies?
What is Hubbles Law? How do distance
measurements tell us the age of the universe?
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How do we measure the distances to galaxies?
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Step 1 Determine size of solar system using radar
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Step 2 Determine distances of stars out to a few
hundred light-years using parallax
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Brightness alone does not provide enough
information to measure distance
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The relationship between apparent brightness
and luminosity depends on distance
Luminosity Brightness
4 pi x
(distance)2 We can determine a stars distance
if we know its luminosity and can measure its
apparent brightness
Luminosity Distance
4 pi x Brightness A standard
candle is an object whose luminosity we can
determine without measuring its distance. We can
use standard candles to measure distances.
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Step 3 Apparent brightness of star clusters
main sequence tells us its distance
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Which kind of stars are best for measuring large
distances?

1. High-luminosity stars
2. Medium-luminosity stars
3. Low-luminosity stars

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Cepheid variable stars are very luminous
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Cepheid variable stars with longer periods have
greater luminosities
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Step 4 The period of a Cepheid variable star
tells us its luminosity. Its luminosity plus
its apparent brightness tells us its distance.
So we can use Cepheid variable stars as
standard candles to measure distances.
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Edwin Hubble, using Cepheids as standard candles,
was the first to measure distances to other
galaxies
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Measuring distances using Cepheids has been a key
mission of the Hubble Space Telescope
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Hubbles extra-sharp vision allows us to observe
individual Cepheid variable stars in galaxies up
to 100 million light-years away
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Clicker Question Youve found a Cepheid variable
star in Galaxy A with a period of 100 days.
You've also found a Cepheid variable star in
Galaxy B with a period of 10 days. Both Cepheids
have the same brightness as seen from Earth.
Which galaxy is farther away from us?

• GET YOUR ANSWER READY
• A) Galaxy A
• B) Galaxy B
• C) Both are the same distance

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Which galaxy is farther away?

1. Galaxy A
2. Galaxy B
3. Both are the same distance

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White-dwarf supernovae can also be used as
standard candles
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Step 5 Apparent brightness of white-dwarf
supernova (another kind of standard candle) tells
us the distance to its galaxy (up to 10 billion
light-years)
Supernova
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In summary, we measure galaxy distances using a
chain of techniques known as the distance ladder
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What is Hubbles Law?
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By measuring velocities and distances of
galaxies, Hubble found that a galaxys velocity
and distance are related in a special way
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The spectral features of virtually all galaxies
are redshifted ? Theyre all moving away from us
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(No Transcript)
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Redshift of a galaxy tells us its distance
through Hubbles Law distance
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Distances of farthest galaxies are measured from
redshifts
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What have we learned?

• How do we measure the distances to galaxies?
• Our measurements of galaxy distances depend on a
  chain of methods. The chain begins with radar
  ranging in our own solar system and parallax
  measurements of distances

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What have we learned?

• What is Hubbles law?
• Hubbles law tells us that more distant galaxies
  are moving away faster. It allows us to determine
  a galaxys distance from the speed at which it is
  moving away from us, which we can measure from
  its Doppler shift.

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Activity 36, page 127-130

• Page 128 has 3 photos of a flat (2-dimensional)
  universe of galaxies, taken at 3 different times.
• Use it to answer the questions in Part I on page
  127.
• Well do those as clicker questions, then do the
  same for Part II.

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2. If you were in one of the other galaxies,
would the MW (Milky Way) galaxy appear to be
moving away from you?

1. Yes
2. No
3. It depends

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4. Are any of the galaxies moving closer to each
other?

1. Yes
2. No
3. Maybe

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5. If we measured radial velocities of other
galaxies, what would we see?

1. All galaxies redshifted
2. All galaxies blueshifted
3. Some galaxies redshifted, some galaxies
   blueshifted

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6. If someone in another galaxy measured radial
velocities of galaxies besides their own, what
would they see?

1. All other galaxies redshifted
2. All other galaxies blueshifted
3. Some galaxies redshifted, some galaxies
   blueshifted

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Part II, page 129 (instructions on page 127)

• Using Figure 2, fill in the blank entries in the
  lower right of Table 1 youre measuring how far
  galaxies A and B moved between images II and III,
  divided by the time elapsed between images II and
  III.
• Then, using the bottom row of Table 1 (which
  youve just filled in), in Figure 3 plot the
  velocities of galaxies A and B as a function of
  their distance.
• Then answer questions 8D, 9, 10 (pages 129-130).

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8D. Figure 3 shows that the farther away a galaxy
is, the ___ it is moving away from the reference
galaxy.

1. Faster
2. Slower
3. Neither speed is independent of distance

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9. How long has galaxy B been travelling if it
started at the origin in Fig. 2, moved at
constant speed, and ended up at its position in
Image III?

1. 2 billion years
2. 4 billion years
3. 8 billion years

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10. Who do you think is correct?

1. Sam
2. Samantha